In conventional fiber glass producing arrangements, molten glass is delivered to an electrically heated bushing which includes a plurality of tips having orifices therethrough for the passage of glass streams. The glass streams are attenuated into fibers which are grouped together as a strand and collected as a package. Generally, the bushings are constructed of an alloy, such as 90 percent platinum and 10 percent rhodium. The tips are painstakingly built up by dropping molten alloy onto a bushine plate, and then drilling the built-up alloy to form the orifices. This arrangement, although commonly used, has a serious limitation in that only a relatively small number of orifices can be provided in a given space.
Another and later approach is to drill small, closely spaced holes in the wall of a metal pipe and deliver molten glass under pressure to the pipe so as to extrude glass through the holes. It has been found that the number of the holes per unit of area of pipe far exceeds the number of tips for a like area of a conventional bushing. Further, the cost of an orifice tube of the type just described is considerably less than the cost of conventional bushing. In such devices, it is customary to provide at least a partial shroud for the orifice tube and to deliver a controlled atmosphere to the shroud to envelope the tube and be discharged near the holes to provide cooling to the filaments and, for certain embodiments protect the tube against oxidation.
By utilizing high pressures up to perhaps several hundred pounds per square inch in the orifice tube, as opposed to conventional one pound per square inch pressure, the fibers may be attenuated from smaller orifices while producing less tension in the filament than was the case with prior, larger, orifice bushings producing an equivalent filament at comparable speeds. Such a high pressure orifice tube fiberizing unit is dislcosed and claimed in U.S. Pat. No. 3,625,025 assigned to the assignee of the present application.
In conventional fiberizing systems, the bushing generally requires from 1500 to 3000 amperes at operating conditions. To provide this power, a step-down tranformer is generally used to provide a high current, low voltage circuit from which the current is supplied to the bushings, with the high-voltage side of the transformer providing the control. Thus in conventional systems the primary side of the transfrmer may include a saturable core reactor or a solid state semiconductor system for regulating the flow of power to the bushing to control the temperature thereof. However, because of the physical size of the components of such a control system, this means of control becomes extremely impractical when considered for load elements such as the orifice tubes describe above, particularly when they are placed in close proximity, for the small size of the orifice tube only requires about one-tenth the space required for a comparable conventional bushing. Further, the orifice tube requires only about one-fifth the power to operate than is required for conventional devices and thus the size of the conventional power supply and controller is not necessary for orifice tube fiberizers.
It has been conventional practice in operating orifice tubes of this type to surround the tubes with a fluid cooling system typically an air supply system to assist in providing close temperature control over the orifices. This is typically accomplished with a plenum chamber constructed and arranged to deliver the air across the orifice tube from both sides from one end to the other. Since some orifices along the length may require more or less cooling fluid to operate effectively, this system has been found to be unsatisfactory in some instances.